Add a fix from Wt 3

If WTimer outlives timerRoot_, this would cause a segfault, so a WTimer
could not reliably be the child of WApplication.

src/Wt/WApplication.C

@@ -381,6 +381,16 @@ std::string WApplication::onePixelGifUrl()
 WApplication::~WApplication()
 {
 #ifndef WT_TARGET_JAVA
+  // Fix issue #5331: if WTimer is a child of WApplication,
+  // it will outlive timerRoot_. Delete it now already.
+  for (std::size_t i = 0; i < children_.size(); ++i) {
+    WTimer *timer = dynamic_cast<WTimer*>(children_[i].get());
+    if (timer) {
+      removeChild(timer);
+    }
+  }
+  timerRoot_ = nullptr;
+
   // First remove all children owned by this WApplication (issue #6282)
   if (domRoot_) {
     auto domRoot = domRoot_.get();

src/Wt/WObject.h

@@ -339,6 +339,7 @@ class WT_API WObject : public Wt::Core::observable
   static unsigned nextObjId_;
 
   friend class EventSignalBase;
+  friend class WApplication;
   friend class WebSession;
 };